The enjoyment of a vast range of human rights, including rights to life, health, food, water, and housing, depend on a healthy and sustainable environment. Today, on World Environment Day, let us remember that those who work to protect the environment are not only environmentalists – they are human rights defenders. And they are increasingly at risk.

As the international demand grows for the exploitation of natural resources in developing countries, so the threats to environmental defenders increase. Those who oppose development projects are often treated as enemies of the state and, all too often, they are targeted for assassination.

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Such murders happen all the time. The deaths of Cáceres and Rhadebe are unusual only in that they received international attention. Mostly when environmentalists are murdered, their deaths go unnoticed beyond their communities. For example, a photographic exhibition in Geneva drew attention last month to the murders of 60 environmentalists over the past two decades in just one country, Thailand. Most had received little or no attention in the international press.

The human rights organisation Front Line Defenders reports that, of the 156 human rights defenders killed in 2015, the largest single group, 45% of the total, were those defending environmental, land, or indigenous rights. Another organisation, Global Witness, has found that an average of two environmental and land rights activists are being killed weekly – and the numbers are getting worse.

The situation is particularly grave in Latin America and south-east Asia, but it affects every region of the world. It is truly a global crisis.

Last year, the international community reached consensus on new sustainable development goals as a roadmap to a more sustainable, prosperous and equitable future. But these goals cannot be met if those on the frontline of sustainable development are not protected.

It is ironic that environmental rights defenders are often branded “anti-development” when, by working to make development truly sustainable, they are actually more pro-development than the corporations and governments that oppose them.

The brave women and men who risk their lives to protect the environment and rights of others should be lauded as heroes. Instead, the authorities typically fail to protect them, to investigate their deaths, or to punish those responsible.

That was a good initial step, but governments must do far more. They have obligations under human rights law to protect environmentalists’ rights of expression and association by responding rapidly and effectively to threats, promptly investigating acts of harassment and violence from all parties including business and non-state actors, protecting the lives of those at risk, and bringing those responsible to justice.

Environmental rights defenders are often branded ‘anti-development’ but they're working to make development sustainable

States must also adopt and implement mechanisms that allow defenders to communicate their grievances, claim responsibilities, and obtain effective redress for violations, without fear of intimidation.

They must take additional steps to safeguard the rights of members of marginalised and vulnerable communities, especially indigenous peoples, whose cultures, identities and livelihoods often depend on the environment and whose lives are particularly susceptible to environmental harm, placing them on the frontline of conflict.

Moreover, international financial institutions should explicitly tie their continuing support for development projects to the implementation of safeguards for human rights, including rights of freedom of expression and association. Multinational businesses should make clear in actions as well as words that they will not undertake projects in countries where these basic protections are not accorded. If they fail to keep their commitments, they should be penalised in their home countries and in the marketplace.

Protecting environmental defenders is not just the right thing to do. It is also the only way to ensure sustainable development.

If we continue to fail to protect those fighting to protect the environment, what new disasters will we face in another 20 years?

•John H Knox is the special rapporteur on human rights obligations relating to the enjoyment of a safe, clean, healthy and sustainable environment. Michel Forst is the special rapporteur on the situation of human rights defenders. VictoriaTauli-Corpuz is the special rapporteur on the rights of indigenous peoples